1. Field of the Invention
The present invention relates generally to flip chip technology and, more particularly, to reworkable circuit board assemblies including a flip chip that can be reworked in the event of failure and a method for fabricating the same.
2. Description of Related Art
The desirability of miniaturization of electronic circuitry and electronic packaging has long been known. Such miniaturization is desirable in electronics employed in various disparate applications including spacecraft, satellites, automobiles, aircraft, consumer electronics, and personal computers to name a few examples. However, heat density problems often arise with increased miniaturization. Specifically, as more electronic circuitry is condensed into smaller and smaller areas to achieve higher packaging density, the heat produced by the condensed circuitry results in a corresponding increase in heat density. Accordingly, it is desirable to provide means for achieving higher electronic packaging density with improved heat transferability.
The current state of the art design in reworkable hermetic packaging comprises multi-chip modules (MCM). MCMs are circuits partitioned into modules of approximately 20 silicon chips. These chips are physically and electronically attached to a substrate which has been assembled inside a ceramic package. After testing, the package is sealed to produce an MCM. Two or three MCMs are then bonded and soldered to each circuit board. Typically, two circuit board assemblies are then bonded to opposite sides of a heat sink to form a processor module.
While such MCMs have achieved reductions in the size and weight of circuitry and packaging, it is desirable to achieve further reductions to achieve higher packaging densities.
Flip chip technology has been proposed as a means to increase packaging density to achieve such size and weight reductions. However, existing flip chip technology suffers from certain drawbacks. For example, prior art flip chips are costly and range from difficult to nearly impossible to rework in the event of failure or other need for chip replacement. Further, while existing flip chip technology can greatly increase packaging density, it also generates high heat density. Moreover, existing flip chips may not be suitable in RF and high speed digital applications due to the lossy underfill material those chips employ.
The need for reworkability is significant. Reworkability permits the replacement of failed chips or chip to circuit board interconnect on circuit boards without replacement of other, properly functioning, components of the board or disposal of the board in its entirety. For example, space processor modules employed in spacecraft often contain tens of ASICs (Application Specific Integrated Circuits). Each such ASIC can cost thousands of dollars. If a flip chip fails in such a processor module, the non-reworkability of the failed chip can result in the disposal of the entire circuit board, possibly wasting the properly functioning ASICs at great expense.
The high cost and non-reworkability of traditional flip chips, as well as the unsuitability of those flip chips for high frequency and high speed applications, is substantially due to the requirement of underfill in those chips. More specifically, traditional flip chips typically have large mismatches between the coefficients of thermal expansion (CTE) of the Integrated Circuits (ICs) and the Printed Wiring Boards the ICs are mounted upon. The use of underfill addresses this problem by reducing the solder joint stress caused by the CTE mismatching. It also acts as a protective coating shielding the chip dies from moisture and foreign corrosive matter. However, the presence of underfill makes it very difficult, if not impossible, to remove and replace a mounted flip chip from a circuit board. The presence of underfill in the existing flip chips also renders those devices unsuitable for high frequency and high speed applications.